A Review of Heavy Metal Contamination in Surface Sediments of the Southern Caspian Sea: Distribution, Ecological Risks, and Source Apportionment Perspectives

Coastal sediments of the southern Caspian Sea act as the ultimate sink for terrestrial inputs, displaying heterogeneous and site dependent heavy metal contamination patterns. Investigations across open coastal areas and semi enclosed basins such as Gorgan Bay have reported measurable concentrations of lead, cadmium, chromium, nickel, zinc, copper, and arsenic, with wide spatial ranges. Although overall risks for most metals are low to moderate, nickel emerges as the most consistent ecological concern in open coastal zones, while local enrichments of cadmium, lead, zinc, and arsenic also raise occasional human health and ecotoxicological concerns. In contrast, Gorgan Bay – a semi enclosed ecosystem – exhibits lower metal concentrations and, based on sediment quality guidelines, shows no adverse biological effects. Hotspots are consistently associated with urban discharges, ports, recreational areas, and river mouths, and the role of sediment texture, organic matter, hydrodynamics, and seasonal variability in secondary metal distribution is clearly evidenced. This review synthesises findings from independent studies over the past two decades, providing a comprehensive picture of the current status of heavy metal pollution in the southern Caspian sediments and outlining knowledge gaps for future research.